1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator in which a crystal unit, an oscillation circuit using the crystal unit and a temperature compensation mechanism for compensating frequency-temperature characteristics of the crystal unit are housed inside a container for surface mounting, and more particularly, to a temperature-compensated crystal oscillator having a data writing terminal for writing temperature compensation data into the temperature compensation mechanism and a crystal inspection terminal for carrying out an operation test of the crystal unit.
2. Description of the Related Arts
For its small size and lightweight as well as high stability of an oscillating frequency against a temperature change, a temperature-compensated quartz crystal oscillator of a surface mount type is widely used as a reference source for a frequency and time, in particular, in a portable electronic apparatus such as a cellular phone. As such a portable electronic apparatus has become smaller in recent years, there is also a need for a smaller surface-mount type temperature-compensated crystal oscillator.
FIG. 1A is a partially sectional front elevation view showing an example of a configuration of a conventional temperature-compensated crystal oscillator of a surface mount type. The illustrated crystal oscillator consists of container body 1, IC (integrated circuit) chip 6 and quartz crystal blank 8. Container body 1 has a flat, substantially rectangular parallelepiped outside shape having short sides and long sides when mounted on a wiring board and viewed from above, and is provided with recesses 20a and 20b on an upper side and a lower side thereof, respectively, resulting in a sectional shape of the letter H. Such container body 1 consists of a laminated ceramic having center layer 1a and frame walls 1b and 1c. Center layer 1a is substantially rectangular and both of frame walls 1b and 1c are frames in shape along the outer circumference of center layer 1a, and respectively laminated onto an upper side and the lower side of center layer 1a. First recess 20a shown on an upper side of the drawing is formed by center layer 1a and frame wall 1b, and second recess 20b shown on a lower side of the drawing is formed by center layer 1a and frame wall 1c. As will be described later, crystal blank 8 which functions as a crystal unit is accommodated in first recess 20a, and IC chip 6 integrating an oscillation circuit and a temperature compensation mechanism is accommodated in second recess 20b. 
FIG. 1A depicts portions of recesses 20a and 20b in a sectional view and center layer 1a is shown in its end face. In addition, FIG. 1B shows a bottom view of center layer 1a alone, showing the back side of center layer 1a, that is, the surface of the party facing second portion 20b. Here, a crystal oscillator described here is a so-called two-room type where a crystal blank and an IC chip are accommodated in different recesses, that is, chambers; beside that, however, there exists a one-room type crystal oscillator where a crystal blank and an IC chip are sealed and enveloped into a same space of a container body. In addition, in general, a crystal blank housed inside a container is referred to as a crystal unit.
On a side surface of container body 1, where corresponds to an end face of center layer 1a, there are provided four data writing terminals 2a to 2d for writing temperature compensation data into the temperature compensation mechanism. Data writing terminals 2a to 2d are arranged so that two of them are provided on each long side of substantially rectangular center layer 1a. In container body 1, mounting electrode 3 used to surface-mount the crystal oscillator on a wiring board is formed in four corners of frame wall 1c forming second recess 20b. The four mounting electrodes provided are a power supply terminal (Vcc), a grounding terminal (E), an output terminal (fo) where oscillation outputs appear and an automatic frequency control terminal (AFC) where automatic frequency control signals are supplied.
On a bottom surface of second recess 20b of container body 1, that is, on the back side of center layer 1a, a plurality of circuit terminals 4 are arranged along the both long sides of center layer 1a, as shown in FIG. 1B. In the drawing, although dashed-dotted lines show the location where IC chip 6 is arranged, each circuit terminal 4 is to be provided corresponding to each of IC terminals to be provided in IC chip 6 as described later. In the drawing, five circuit terminals 4 are provided in a row along each long side. Total of four circuit terminals 4 provided in ends of each row, in other words, respective circuit terminals 4 provided at four corners are a power supply terminal (Vcc), a grounding terminal (E), an output terminal (fo) and an automatic frequency control terminal (AFC), respectively, and are electrically connected to corresponding mounting electrodes 3 by conductive paths 5 each formed in a straight line on center layer 1a. On each row of the circuit terminals, the circuit terminals at the center of the rows are crystal circuit terminals 4×1, 4×2 electrically connected to crystal blank 8 as described below. Remaining six circuit terminals excluding circuit terminals 4 corresponding to mounting electrodes 3 and crystal circuit terminals 4×1, 4×2 are writing circuit terminals 4a to 4d and are provided two for each of the rows. Writing circuit terminals 4a to 4d are electrically connected to data writing terminals 2a to 2d, respectively, by straight conductive paths 5 provided on the back side of center layer 1a. 
Moreover, a pair of crystal inspection terminals X1, X2 are provided on the back side of center layer 1a. Crystal inspection terminals X1, X2 are electrically connected to crystal circuit terminals 4×1, 4×2 respectively, by straight conductive paths 5 provided on the back side of center layer 1a. 
IC chip 6 is substantially rectangular and has an oscillation circuit using crystal blank 8 and a temperature compensation mechanism for compensating frequency temperature characteristics of crystal blank 8 which are integrated on a semiconductor substrate. These oscillation circuit and temperature compensation mechanism are formed on one main surface of a semiconductor substrate through a general semiconductor device fabrication process. Therefore, one of the two main surfaces of IC chip 6 on which the oscillation circuit and the temperature compensation mechanism are formed in the semiconductor substrate will be referred to as a circuit forming surface.
A plurality of IC terminals 7 are arranged on the circuit forming surface along the both long sides of the circuit forming surface. Each of IC terminals 7 corresponds to each of circuit terminals 4 on center layer 1a of container body 1. IC terminals 7 are electrically connected to a circuit inside IC chip 6. FIG. 2A illustrates an arrangement of IC terminals 7 and is a perspective view of IC chip 6 viewed from the opposite side of the circuit forming surface in order to clarify the correspondence with circuit terminal 4. Among arranged IC terminals, the IC terminals arranged at four corners are a power supply (Vcc) terminal and a grounding (E) terminal for supplying IC chip 6 with electric power, an output (fo) terminal where oscillation outputs from an oscillation circuit appear, and an automatic frequency control (AFC) terminal for supplying the oscillation circuit with an automatic frequency control signal. Two of the remaining six IC terminals are crystal IC terminals 7×1, 7×2 and other four of them are IC terminals 7a to 7d. Crystal IC terminals 7×1, 7×2 correspond to crystal circuit terminals 4×1, 4×2 of center layer 1a, respectively, and are used to electrically connect to the oscillation circuit inside IC chip 6 so that crystal blank 8 is incorporated electrically inside an oscillation closed loop of the oscillation circuit. Writing IC terminals 7a to 7d correspond to writing circuit terminals 4a to 4d of center layer 1a, respectively, and are electrically connected to the temperature compensation mechanism inside IC chip 6 and used to write the temperature compensation data to the temperature compensation mechanism.
Each IC terminal 4 is fixed onto circuit terminal 7 of center layer 1a by flip chip bonding, for example, ultrasonic thermal compression bonding by means of bump 18. In this way, IC chip 6 will be secured onto the bottom surface of second recess 20b of container body 1 so that IC chip 6 is housed inside recess 20b. At that time, the longitudinal direction of IC chip 6 corresponds with the longitudinal direction of center layer 1a. 
Crystal blank 8 made of a substantially rectangular AT-cut quartz crystal blank, for example, as shown in FIG. 2B, and excitation electrodes 9 are formed on the both main surfaces of crystal blank 8, respectively. Extending electrodes 10a, 10b extend toward the both sides of an end of crystal blank 8 from a pair of excitation electrodes 9. On a bottom surface of first recess 20a, that is, on the front side of center layer 1a, a pair of crystal retaining terminals (not shown) is provided. The pair of crystal retaining terminals is electrically connected to crystal circuit terminals 7×1, 7×2 by conductive paths (not shown) provided inside center layer 1a. Crystal blank 8 will be fixed and held inside first recess 20a by fixing a pair of extending electrodes 10a, 10b onto the crystal retaining terminals, respectively, with, for example, conductive adhesive and the like in a location where these extending electrodes are led out. In addition, by fixing crystal blank 8 with conductive adhesive and the like, crystal blank 8 will be electrically connected to the oscillation circuit inside IC chip 6 and be incorporated inside the oscillation closed loop.
After crystal chip 8 has been fixed, metal cover 11 is joined to opening side of first recess 20a of container body 1 by applying seam welding or the like so that crystal chip 8 will be hermetically sealed in first recess 20a. 
In fabricating such a surface mount type temperature-compensated crystal oscillator, crystal blank 8 is first hermetically sealed in first recess 20a to configure a crystal unit, and thereafter, oscillation characteristics, such as crystal impedance (CI), of crystal blank 8 as a crystal unit such are measured with a pair of crystal inspection terminals X1, X2 provided on the bottom surface of second recess 20b, that is, on the back side of center layer 1a. Specifically, a measurement probe is brought into contact with crystal inspection terminals X1, X2 to measure the oscillation characteristics. Those determined to be out of standards in oscillation characteristics is discarded as defective products, and for those determined to be good products, IC chip 6 is fixed onto the bottom surface of second recess 20b and temperature compensation data are written into IQ chip 6 from data writing circuit terminals 2a to 2d provided on the exterior of container body 1. Thereafter, however not shown here, in order to protect a circuit forming surface of IC chip 6, resin as “under-fill” is injected inside second recess 20b and thereby temperature-compensated crystal oscillator is completed.
However, in the above described temperature-compensated crystal oscillator, crystal inspection terminals X1, X2 provided on the bottom of second recess 20b in container body 1 will be covered with IC chip 6 after IC chip 6 has been fixed onto second recess 20b and will also be covered with under-fill resin. Consequently, after completion as a product, oscillation characteristics for crystal unit alone will not be measurable. In that case, even if failure might occur such as oscillation defects and the like in a temperature-compensated crystal oscillator after a product is shipped, for example, oscillation characteristics for the crystal unit alone cannot be analyzed and therefore it is difficult to discover the cause for the oscillation defects.
In addition, if temperature-compensated crystal oscillators are further downsized to, for example, not larger than 3.2×2.5 mm in its footprint, the size of a bottom of recess 20b in container body 1 will also be small so that it will become impossible to form crystal inspection terminals X1, X2 of a sufficient size on the bottom of second recess 20b. Due to a probe of a measuring device for measuring oscillation characteristics, crystal inspection terminals X1, X2 require such a size as not less than 0.6×0.6 mm, for example; however, if temperature-compensated crystal oscillators are further downsized, the crystal inspection terminal will be smaller than this size so that it will be impossible to make definite measurement on oscillation characteristics.
Therefore, also in a temperature-compensated crystal oscillator of two-room type as described above, it is conceivable to arrange crystal inspection terminals on an outer side surface of a container body as implemented in case of a temperature-compensated crystal oscillator of one-room type (see US 2006/0055478A1, for example). In that case, in center layer 1a, conductive paths will be provided to electrically connect to crystal circuit terminals 4×1, 4×2 from the crystal retaining terminals to which crystal blank 8 is fixed through crystal inspection terminals X1, X2 provided on an end face of center layer 1a. In consideration of ongoing downsizing of crystal oscillators, the side surface area of container body 1 will be small and, therefore, it is difficult to provide a total of three terminals, or crystal inspection terminals and data writing terminals, on one exterior surface. Therefore, two data writing terminals may remain on each side surface including a long side of center layer 1a among the side surfaces of container body 1, and crystal inspection terminals may be provided onto respective side surfaces including short sides of center layer 1a among side surfaces of container body 1. FIG. 3 shows terminal arrangement on a back side of center layer 1a in case of thus having formed crystal inspection terminals X1, X2 on both end faces of center layer 1a. 
However, when crystal inspection terminals are arranged in this way, and considering the arrangement of IC terminal 7 in IC chip 6 described above, conductive path 5 connecting each of crystal inspection terminals X1, X2 provided on the both end faces in the direction of a long side from crystal circuit terminals 4×1, 4×2 will be long on center layer 1a. This means that line capacitance, stray capacitance and the like in the conductive path directly connected to crystal chip 8 increases, and consequently gives rise to such a problem of complicated design of crystal oscillators or complicated quality management because, for example, oscillating frequency may largely deviate from the designed value.